Contents

Symbol and abbreviations

The BIPM symbol for the tonne is 't', adopted at the same time as the unit in 1879.[5] Its use is also official for the metric ton in the United States, having been adopted by the United States National Institute of Standards and Technology (NIST).[6] It is a symbol, not an abbreviation, and should not be followed by a period. Use of majuscule and minuscule letter case is significant, and use of other letter combinations is not permitted and would lead to ambiguity. For example, 'T', 'MT', 'mT', 'Mt', 'mt' are the SI symbols for the tesla, megatesla, millitesla, megatonne (one teragram), and millitonne (one kilogram) respectively. If describing TNT equivalent units of energy, one megatonne of TNT is equivalent to approximately 4.184 petajoules.

Origin and spelling

In French and English, tonne is the correct spelling. It is usually pronounced the same as ton (/tʌn/), but the final "e" can also be pronounced, i.e. "tunnie" (/ˈtʌnɪ/).[7] In Australia, it is also pronounced /tɒn/.[8] In the United States, metric ton is the name for this unit used and recommended by NIST;[6] an unqualified mention of a ton almost invariably refers to a short ton of 2,000 pounds (907 kg), and tonne is rarely used in speech or writing. Both terms are acceptable in Canadian usage.

Before metrication in the UK, the unit used for most purposes was the Imperial ton of 2,240 pounds avoirdupois or 20 hundredweight (usually referred to as the long ton in the US), equivalent to approximately 1,016 kg, differing by about 1.6% from the tonne. The UK Weights and Measures Act 1985 explicitly excluded from use for trade certain imperial units, including the ton, unless the item being sold or the weighing equipment being used was weighed or certified prior to 1 December 1980, and even then only if the buyer was made aware that the weight of the item was measured in imperial units.[9][full citation needed][10][11]

Ton and tonne are both derived from a Germanic word in general use in the North Sea area since the Middle Ages (cf. Old English and Old Frisiantunne, Old High German and Medieval Latintunna, German and Frenchtonne) to designate a large cask, or tun.[12] A full tun, standing about a metre high, could easily weigh a tonne. An English tun (an old wine cask volume measurement equivalent to approximately 954 litres) of wine has a relative mass of approximately 954 kg if full of pure water, a little less for wine.

The spelling tonne pre-dates the introduction of the SI in 1960; it has been used with this meaning in France since 1842,[13] when there were no metric prefixes for multiples of 106 and above, and is now used as the standard spelling for the metric mass measurement in most English-speaking countries.[14][15][16][17] In the United States, the unit was originally referred to using the French words millier or tonneau,[18] but these terms are now obsolete.[2] The Imperial and US customary units comparable to the tonne are both spelled ton in English, though they differ in mass.

Derived units

For multiples of the tonne, it is more usual to speak of thousands or millions of tonnes. Kilotonne, megatonne, and gigatonne are more usually used for the energy of nuclear explosions and other events in equivalent mass of TNT, often loosely as approximate figures. When used in this context, there is little need to distinguish between metric and other tons, and the unit is spelt either as ton or tonne with the relevant prefix attached.[21]

*The equivalent units columns use the short scale large-number naming system currently used in most English-language countries, e.g. 1 billion = 1,000 million = 1,000,000,000.†Values in the equivalent short and long tons columns are rounded to five significant figures. See Conversions for exact values.ǂThough non-standard, the symbol "kt" is also used for knot, a unit of speed for aircraft and sea-going vessels, and should not be confused with kilotonne.

Alternative usage

A metric ton unit (mtu) can mean 10 kg (approximately 22 lns) within metal (e.g. tungsten, manganese) trading, particularly within the US. It traditionally referred to a metric ton of ore containing 1% (i.e. 10 kg) of metal.[22][23]
The following excerpt from a mining geology textbook describes its usage in the particular case of tungsten:

Unit of force

Like the gram and the kilogram, the tonne gave rise to a (now obsolete) force unit of the same name, the tonne-force, equivalent to about 9.8 kilonewtons: a unit also often called simply "tonne" or "metric ton" without identifying it as a unit of force. In contrast to the tonne as a mass unit, the tonne-force or metric ton-force is not acceptable for use with SI, partly because it is not an exact multiple of the SI unit of force, the newton.

^"Canada Gazette". Government of Canada. 1998–2007. Retrieved 2010-02-13. The Corporation shall pay to producers selling and delivering wheat produced in the designated area to the Corporation the following sums certain per tonne basis...

^The Oxford English Dictionary 2nd ed. gives both megaton and megatonne and adds "The unit may be calculated in either imperial or metric tons; the form megatonne generally implies the metric unit". The use for energy is the first definition; use for mass or weight is the third definition.